This invention relates to a gas-discharge lamp which makes use of a so-called dielectric barrier discharge. To this end, a discharge vessel which is at least partially transparent and filled with a gas filling has at least one anode and at least one cathode. The electrodes have a geometry in the form of strips, that is to say they are in the form of strips at least in places. However, they may also have more complex shapes, for example with branches. In the case of a dielectric barrier discharge, at least one of the electrodes, the anode for unipolar operation, must be covered with a dielectric layer.
However, in the context of this application, the terms anode and cathode should not be regarded as limiting the invention to unipolar operation. In a bipolar case, there is no difference between anodes and cathodes, so that the statements for one of the two electrode groups then apply to all electrodes.
1. Background Art
Lamps with a dielectric barrier discharge are known in the prior art, particularly for back lighting of flat screens. This application area will not be described in detail here.
With respect to a preferred embodiment of the invention described further below, reference is made, as prior art, to Hella-Lichttechnik R and D Review 1996 (08/96), page 119, and to EP 0 813 996 A2. This prior art includes the idea of improving the warning function of a brake warning light by varying the illuminated area, for example varying the illuminated length of the brake warning light.
2. Description of the Invention
This invention is based on the technical problem of extending the application options for gas-discharge lamps using a dielectric barrier discharge. According to the invention, this problem is solved by a gas-discharge lamp having a discharge vessel filled with a gas filling, having at least one anode in the form of a strip and having at least one cathode in the form of a strip, which are arranged essentially parallel to one another, at least in places, and having a dielectric layer at least between the anode and the gas filling, characterized in that in the region of its essentially parallel profile, the electrode arrangement is at least partially inhomogeneous along its length in a form which varies a maintaining voltage.
Furthermore, the invention for solving this problem relates to a method for actuating such a gas-discharge lamp having pulsed real-power injection, in which a maintaining voltage for the lamp is varied by varying at least one time parameter of the supply power.
Finally, one particular solution to this problem according to the invention results from an apparatus for indicating a braking deceleration of a motor vehicle or two-wheeled vehicle having such a lamp, a braking deceleration sensor and a control unit which is supplied with a signal from the braking deceleration sensor and actuates the lamp.
The basic idea of the invention is to design the electrode system of a lamp with a dielectric barrier discharge such that inhomogeneous discharge preconditions exist along at least a part of the length of the electrodes. In this case, the aim is to monotonically vary a maintaining voltage for the discharge in places, at least in terms of an effective mean value. This maintaining voltage may be, in particular, a minimum maintaining voltage which in this case does not correspond to the starting voltage of an individual discharge, but is the minimum voltage which allows a discharge structure to be maintained at a specific point in the electrode arrangement.
In the case of the pulsed real-power injection, which is considered in a preferred manner here, the restarting of an individual discharge in the residual ionization that still remains after one of the regular brief interruptions in the real-power injection, that is to say those brief interruptions which occur in continuous operation of the light, does not mean restarting. In fact, restarting means switching on the lamp again without the gas filling having any specific residual ionization.
A major advantage of a gas-discharge lamp with a dielectric barrier discharge over conventional gas-discharge lamps is the positive current/voltage characteristic. By virtue of the unambiguous relationship between the current and voltage, this allows a change in the supply voltage to lead to a change in the illuminated length of the gas-discharge lamp with a dielectric barrier discharge, and thus to a change in the lamp current. In conventional fluorescent lamps, this is prevented by a negative differential resistance in the current/voltage characteristics.
If the minimum maintaining voltage is now varied over a portion of the length of the electrode arrangement in the manner according to the invention, then it is possible, during operation, to control the portion of this length section with the monotonically varying minimum maintaining voltage over which discharges burn, by adjusting and varying the power supply, in particular its voltage. The illuminated length section is thus adjusted.
There are various options for the minimum maintaining voltage of such an inhomogeneous electrode arrangement to be monotonically dependent on position. A first option is to vary the distance between the electrodes that governs the discharge. The larger the gap becomes, the greater is the minimum maintaining voltage required to maintain a discharge across this distance.
On the other hand, the difference between the starting voltage and the minimum maintaining voltage can be explained in that a discharge to a specific point in the electrode arrangement with a specific gap can always start in an adjacent region with a shorter gap and can move into the region in which the available voltage is just still sufficient for the discharge. This is due to the fundamental phenomenon that the discharge structures are wherever possible distributed over the available electrode surfaces since, for a dielectric barrier discharge, the greater available area on the dieletrically coated electrode provides better high-frequency conductivity and thus a reduced voltage drop across the dielectric.
On the other hand, there are also structures in which the movement of individual discharge structures between points with a gap which is sufficiently short to start a discharge and points at which the gap is only sufficiently short to maintain a discharge that has been started at some other point is not directly possible. For example, in the case of the invention, it is possible to provide the electrodes with projections (which are known per se) for physical localization of individual discharges. These projections may be, for example, small tabs on one or both electrodes, between which the discharge is maintained. The critical distance for starting and maintaining a discharge is then the distance between the tip of such a tab and the opposite electrode, or between the tips of two opposite tabs. It is obvious that, in this case, the discharge structures cannot move continuously and a voltage which is sufficient to maintain the discharge between the projections must first of all be available for a further movement step (to the next projection) to take place. In an extreme case, the situation may thus even arise where the maintaining voltage mentioned in claim 1 corresponds to the discharge starting voltage and not to the minimum maintaining voltage. Compromises between these extreme situations are, of course, also feasible.
Furthermore, it can be seen from the example with the electrode projections that the discharge characteristics of the electrode arrangement need not necessarily be varied continuously or monotonically. However, for various applications (which will be described in more detail in the following text), the discharge characteristics should be monotonic functions of position over a certain region of the electrode arrangement at those points which always support discharges, that is to say, for example, at the tips of the projections.
A further option for varying the discharge voltage is the dependency of the anode width on position. On the one hand, the anode width influences the surface area of the anode available for the discharge, and thus the current flowing in the discharge. The discharge current in turn governs the residual ionization of the gas filling that remains at the end of a dead-time period between two real-power pulses, and this governs the probability of restarting. On the other hand, when the discharge current is distributed over a larger anode area, this results in a smaller voltage drop across the dielectric, and thus in a stronger electrical field in the gas filling.
The anode width can, of course, be varied both for essentially xe2x80x9csmoothxe2x80x9d electrodes and in conjunction with the described electrode projections. Furthermore, the thickness of the dielectric can also be varied, thus allowing the discharge current and the electrical field in the gas filling to be influenced in an analogous manner.
The examples explained so far relate to inhomogeneities in the electrode arrangement in order to influence a discharge voltage. In a gas-discharge lamp designed in such a way, it is thus possible to control the length sections of the electrode arrangement in which discharges are maintained, and where they are not, by varying the voltage of a power supply, for example the voltage in the real-power pulses from a pulsed power supply.
However, in such a gas-discharge lamp, it is also possible to adjust the length section where discharges occur using other electrical supply parameters. In particular, the minimum maintaining voltage of the lamp depends on specific time parameters of a supply power with pulsed real-power injection. One possible time parameter is the dead time between the real-power pulses. The longer this dead time is chosen to be, the less is the residual ionization remaining at the end of the dead time and thus the lower is the probability of restarting, or the higher is the voltage required for restarting (within continuous light operation, that is to say between separate real-power pulses.
Another possible time parameter is the time derivative of the voltage rise, that is to say the gradient of the voltage rise, at the start of a real-power pulse. Initially, this option (as, in principle, are all the inventive measures described so far, as well) is an empirical result of the development work by the inventors. One possible explanation could be that, as the voltage rise becomes steeper and the weighting of the high-frequency Fourier components of the voltage waveform becomes greater, the high-frequency conductivity of the dielectric in particular is improved and thus, as already explained, the electrical field that exists in the gas filling becomes stronger.
One preferred application of the gas-discharge lamp according to the invention is a lamp for a bar display. For this purpose, the discharge vessel has an elongated shape, for example a tubular shape, and the strip arrangement of the electrodes extends at least along a portion of the elongated shape. In this case, the already described inhomogeneity of the electrode arrangements for a bar display lamp is chosen such that the discharge voltage is position-dependent along the length of the bar display, or of a portion of it. By adjusting the voltage of the power supply or by adjusting the described time parameters, it is now possible to set the length of the bar display lamp that illuminates. Quantitative information content can thus be provided by such a bar display, for example relating to specific technical parameters for an electronic appliance or an electrical system, and conventional LED bar displays or illuminated analogue instruments can be replaced. This is of particular interest in applications in which the brightness of the display plays a significant role.
In the case of electrodes which are to some extent xe2x80x9csmoothxe2x80x9d, the bar display is in this case virtually continuous; if the inhomogeneity has a stepped configuration or if the described projections are used, the information from the bar display can also, however, be conveyed discretely, that is to say non-continuously between different stages of illuminated lengths.
A tubular shape of the discharge vessel is also advantageous, for example, for a further application of the gas-discharge lamp according to the invention and is of particular interest. In this case, the lamp according to the invention is used as a brake warning light on a vehicle, in particular a motor vehicle or a two-wheeled vehicle. Such a brake warning light combines the warning and signalling function of a conventional brake warning light and also provides a graduated indication of the severity of the deceleration, so that traffic travelling behind can react in an appropriate manner.
For this purpose, the brake warning light is connected to a control unit, which receives a signal from a braking deceleration sensor. The braking deceleration sensor may be a dynamic deceleration sensor, for example a piezoelectric deceleration sensor. However, a kinematic device is also possible, which calculates the braking deceleration from the rate of change of the speed of travel. The speed of travel may be derived, for example, from an actuation signal for a vehicle tachometer or for an on-board computer.
A further option is indirect measurement of the braking deceleration via the braking device in the motor vehicle or two-wheeled vehicle. For example, it is possible to detect the brake pedal pressure or the contact pressure or tensile force on a brake lever. It is also particularly simple to detect the position or deflection of the brake pedal or brake lever. These indirect variants of deceleration sensors also have the advantage that they lead to the brake warning light being fully illuminated, for example when attempting hard braking on slippery ground, even though the actual physical deceleration is, possibly, rather low. This ensures an unrestricted warning function in such critical road traffic situations.
On the other hand, dynamic and kinematic (direct) deceleration sensors can lead to full response of the brake warning light in situations in which the driver scarcely operates the braking system, or does not operate it at all, for example in the event of a rear-impact collision which the driver identifies too late. Combinations of both options are, of course, also feasible, combining the corresponding advantages.
With regard to the configuration of the brake light itself, a maximum warning function is provided if this light extends essentially over the entire vehicle width, in particular of a motor vehicle. If the illuminated part becomes larger outwards to the left and right from the center of the motor vehicle as the deceleration increases, the vehicle width provides a reference length and, in normal braking manoeuvres where the response of the brake warning light is limited, this provides a direct similarity between the appearance of the third brake warning lights which are currently being introduced into road transport.
On the other hand, the complementary geometry, in which the illuminated region of the brake warning light extends increasingly from the left and right on the outside towards the center has the advantage that the distance between the outer limits of the illuminated region provides a reference scale even when visibility is poor. Traffic travelling behind can thus relate the length of the overall illuminated region to these outer limits. In the complementary case, such a reference scale is provided only if the width of the brake warning light or of the motor vehicle can be identified because the environmental brightness is sufficiently good, or from other rear lights.
In order to formulate the already mentioned elongated form of the electrode arrangement and the inhomogeneity somewhat more specifically, it is preferable in this invention for this inhomogeneity to extend along a distance which is considerably greater than the discharge gap between the relevant electrodes, if the variation of the discharge gap is considerably greater than the minimum discharge gap. In particular, this distance should in this case be longer than twice, and preferably longer than five times, the (minimum) discharge gap.
Particularly with respect to the already mentioned application areas for a bar display and a brake warning light, it is in this context furthermore preferable for the length of the inhomogeneity to make up a considerable portion of the length of the discharge lamp, at least a considerable portion of the length of the approximately parallel electrode profile. In this case, the major applications should be born in mind, in which the inhomogeneity - for monotonic variation of the maintaining voltage - extends over roughly the entire length of the parallel electrode profile, or over approximately half of it, in which case the other half may be chosen to have mirror-image symmetry. To this extent, length elements of at least one third, and preferably 40% or 45% of the length of the approximately parallel profile, are preferred.